1. Field of the Invention
The present invention relates to a ball step gauge that is a standard gauge for calibrating, measuring or examining the accuracy of the length measurement of a coordinate measuring machine, a machine tool, a machining center, etc.
2. Discussion of the Background
A previous technology coordinate measuring machine is a machine for measuring the dimensions and shape of a workpiece using coordinate points X, Y and Z interspersed in a three-dimensional space with the aid of a computer. To be specific, the workpiece under measurement placed on a table and a probe attached to the end of a Z-axis of the measuring machine are relatively moved in the three-dimensional directions of X, Y and Z, moments of contact of the probe with the workpiece are ascertained, coordinate values in the directions of the moving axes are read using the moments as electrical triggers, and the dimensions and shape of the workpiece are measured using the computer.
Generally, coordinate measuring machines are required to measure a workpiece with particularly high accuracy. In order to guarantee high-accuracy measurement, a coordinate measuring machine has to be subjected to accuracy examination frequently, and values obtained by the measurement with the coordinate measuring machine are calibrated using the accuracy examination results as calibration values or the coordinate measuring machine is finely adjusted. This accuracy examination requires use of a gauge as the standard. The gauge is required to enable evaluation of values detected by the probe of the coordinate measuring machine when it is moved three-dimensionally.
A first important target for researchers was how the error of each axis in the coordinate measuring machine should be measured. Therefore, a gauge was first invented for the purpose of measuring such errors of the coordinate measuring machine. It is now widely known that, fundamentally, the errors should be measured by measuring a sphere or spheres. For this reason, research has turned to a second target of determining how the sphere or spheres should be arranged in a gauge for measurement and evaluation. Various attempts have been made to arrange the spheres in one same plane and arrange them in the three-dimensional space.
As a ball gauge using spheres, ball step gauges each with plural spheres arranged linearly as shown in FIG. 9 and FIG. 10 have been widely used. The ball step gauge 86 shown in FIG. 9 comprises a gauge frame body 80 in which three circular holes 81 are formed, a ball receiving portion 82 formed at the center bottom of each circular hole, probe insertion grooves 83, 83 formed around and across the ball receiving portion and opposed in the lengthwise direction of the gauge frame body, probe insertion grooves 84, 84 formed opposite to each other in the direction orthogonal to the direction in which the first mentioned probe insertion grooves are disposed, and a highly precise spherical ball 85 fixed on the ball receiving portion 82.
A coordinate measuring machine is calibrated, for example, using this ball step gauge 86 by placing the ball step gauge 86 on and fixing it to a table, bringing the probe of the coordinate measuring machine into contact with at least four points on the outer periphery of a ball 85 that is, for example, a rightmost one in the figure to measure the center position of that ball by computation, and successively measuring the positions of the remaining balls in the same manner. The distance between adjacent balls obtained from the ball positions has been calibrated by a high-precision coordinate measuring machine. The results of measurement are compared with the calibrated distance value to calibrate the coordinate measuring machine.
In another prior art ball step gauge shown in FIG. 10(a), three supports 93 each having a ball 92 fixed thereto are fixed onto a base frame body 91. Each support 93 is partially chipped off to have a leaf-spring effect, so that the ball 92 supported on the tip of the support is laterally swingable. Between the center ball and each of the right and left balls is disposed a connection pipe 94 whose opposite end faces in contact with the balls have a shape conforming to the outer shape of the balls 92. A pair of support frames 96 rise from the opposite ends of the base frame body 91, and a pressure application pipe 95 interposes between each of the right and left balls and each of the support frames. The connection pipes are pressed by screws 97 driven into the support frames. The distance between adjacent balls can be determined in accordance with the effective length of the connection pipes 94. The end faces of the pressure application pipes 95 in contact with the balls 92 have a shape conforming to the outer shape of the balls 92, similarly to the end faces of the connection pipes 94.
FIG. 10(b) is a plan view showing a part of FIG. 10(a) and, as shown, probe insertion grooves 98 are formed at portions at which the connection pipes 94 and pressure application pipes 95 abut on the balls 92 and have the same function as the probe insertion grooves 83 of the ball step gauge 86 shown in FIG. 9. The distance between adjacent balls 92 of the ball step gauge 99 has also been calibrated by a high-precision coordinate measuring machine. The ball positions are successively measured in the same manner as in the ball step gauge shown in FIG. 9, and the results of measurement are compared with the calibrated distance value to enable the ball step gauge 99 to calibrate a coordinate measuring machine.
In the ball step gauges shown in FIG. 9 and FIG. 10, since the distance between adjacent balls is measured by a high-precision coordinate measuring machine, the precision of the distance is somewhat high. However, when a difference in temperature is produced between the upper and lower sides and/or between the right and left sides of the frame body due to external thermal turbulence, the temperature difference gives rise to thermal expansion of the frame body. As a result, the frame body exhibits a bimetallic effect and is bent to lower the precision of the ball step gauge.
The present invention can solve the above problems and aims at providing a ball step gauge in which dimensional changes in the distance between adjacent balls are less liable to entail even when a framework is bent by its own bimetallic effect resulting from thermal expansion caused by the temperature difference between the upper and lower sides and/or between the right and left sides of the framework due to external thermal turbulence, and are very small even when the framework serving as an elastic support beam is elastically deformed by its own static load.
The ball step gauge of the present invention comprises a gauge framework that is H-shaped in cross section, a plurality of holes formed at predetermined intervals in a horizontal frame of the gauge framework in an axial direction of the horizontal frame, a plurality of grooves formed around each of the holes, and a plurality of balls inserted under pressure in the holes, with centers of the balls existing on a neutral axis of moment of inertia of area of the gauge framework.
As described above, since the centers of all the balls exist on the neutral axis of the second moment of area of the gauge framework, the dimensions of the ball intervals are difficult to change even when the framework is bent by exterior thermal turbulence. Furthermore, since the framework is an elastic support beam, it is elastically deformed as a beam by its own static load. However, the changes in the ball intervals can be made small.
Moreover, by providing restraint surfaces for preventing a ball interval measuring interferometric stepper from rolling, in parallel to the axis along which the balls are arranged, an accurate ball step gauge can be obtained.